An agent that has stopped and an agent with nothing to report look identical from the outside. That is what makes this the most expensive failure in the category. Here are the reasons agents actually stop, roughly in order of how often they happen, and what each one looks like when you go looking.
Almost always one of six things: the process crashed and nothing restarted it, the host rebooted, an API key expired or was revoked, the model account ran out of credit, a tool or website the agent depends on changed, or the schedule never fired because the machine was asleep. The first step is checking whether the process is alive, not rewriting the prompt.
Most common
The process died and nothing restarted it
Most expensive
Silent stopping, unnoticed for days
Most misdiagnosed
Model drift blamed on the prompt
First check
Is the process alive, not is the prompt right
Symptom to cause
What you see
Most likely cause
First thing to check
Nothing at all, no logs
Process died, or schedule never fired
Is the process alive
Stopped overnight
Host rebooted
System uptime
Fails every run at a precise time
Credential expired or revoked
Logs, for an auth error
Runs but produces nothing
Out of credit
Provider billing page
Quality got worse, gradually
Model or dependency changed
Provider and API changelogs
One step fails, rest works
A tool or site changed
That tool in isolation
1. The process died and nothing brought it back
The single most common cause, and the most mundane. Long-running processes get killed: the kernel reclaims memory under pressure, an unhandled exception escapes, a dependency segfaults. On a laptop this is invisible. On a server without a restart policy, it is equally invisible.
Symptom: everything worked, then nothing, with no error anywhere you look.
Check: is there still a process? On a server, is the service active rather than merely enabled?
Fix: a supervisor that restarts on failure, and an alert when it does so repeatedly. A restart loop is a different problem wearing the same mask.
2. The host rebooted
Automatic security updates reboot machines. Power blips restart Mac Minis. Cloud providers migrate instances. If the agent was started by hand in a terminal, none of it survives.
Symptom: the agent stopped at a time nobody was working, often overnight.
Check: system uptime. If it is shorter than the outage, you have your answer.
Fix: the agent has to start on boot, not when someone types a command.
3. A credential expired, rotated, or was revoked
Provider keys are not permanent. They get rotated on a schedule, revoked after a leak scan, or invalidated when a team member who created them leaves. The agent then fails on every run, usually with a 401 buried in a log nobody reads.
Symptom: consistent failure starting at a precise moment, on every run.
Check: the logs, for an auth error rather than a model error.
Fix: store credentials in one place so rotation is one edit, and make auth failures loud rather than another retry.
4. The money ran out
A prepaid balance hit zero, a card expired, or a spending limit you set months ago was reached. The agent is alive and correctly configured, and every model call fails.
Symptom: the agent runs, logs an attempt, and produces nothing useful.
Check: the provider's billing page before anything else.
Fix: a balance you can see, and an alert before zero rather than at zero.
5. The world changed underneath it
This is the one that gets misdiagnosed as a prompt problem. A model was deprecated. An API changed a field name. A site the agent reads redesigned its markup. The agent is running perfectly and doing the wrong thing.
Symptom: output quality degraded rather than stopped, or one specific step now fails.
Check: the changelog of whatever it depends on, and whether the failure is one tool or all of them.
Fix: pin what you can, and treat a dependency change as an event worth re-testing after.
6. The schedule never fired
The agent did not fail. It was never asked to run. A cron job on a sleeping laptop does not queue up and catch up later; the moment simply passes.
Symptom: no logs at all for the run, as opposed to a failed run.
Check: whether anything was recorded for that timestamp.
Fix: run the schedule somewhere that does not sleep, and record every trigger even when the work is a no-op.
The detection problem underneath all six
Notice that five of the six are easy to fix and hard to notice. That asymmetry is the actual problem. An agent that reports nothing on a quiet day and an agent that has been dead since Tuesday produce the same silence, so the only reliable signal is a heartbeat: proof the agent ran, sent whether or not it had anything to say.
Do not infer it from output, because an agent with nothing to report is indistinguishable from a dead one. Use a heartbeat: have the agent record every scheduled run, including runs where it found nothing, so the absence of a record is itself the alarm.
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